Circularly polarized omnidirectional cone mounted spiral antenna



June s, 1965 J D. DYSON ETAL 3,188,643

v CIRCULARLY P'OLARIZED OMNIDIRECTIONAL CONE Filed Dec. 29, 1960 MOUNTEDSPIRAL ANTENNA 2 Sheets-Sheet 2 CIRCULARLY PGLARIZED UMNIDIRECTIGNAL fCNE MUN'IED SPL ANTENNA John D. Dyson and Paul E. Mayes, Champaign,Iii.,

y assignors to The University of Illinois Foundation, a

non-profit organization of Iliinois Filed Dec. 29, 196i), Ser. No.'79,432 It) Claims. (Cl. 343-895) This invention relates to antennasand, more particularly, it relates to antennas having conical beam oromnidirectional radiation patterns that are essentially independent offrequency over wide bandwidths.

' It is known that if the shape of an antenna were such that it could bespecified entirely by angles, the antenna would make an ideal broadbandradiator, since its operation would. be theoretically independent offrequency. All antennas which meet this criterion, however, are infinitein extent, so that it is necessary to specify at least one length for anantenna of finite size. By making this one length very large comparedwith the wavelength of operation of a given antenna, it is possible insome cases to achieve antenna performance which is practicallyindependent of Wavelength over wide bandwidths.

An antenna which can be made to have a very wide band of operations inwhich performance is independent of wavelength is the balancedequiangular spiral antenna. The one specified length for this antenna,the

arm length, need not be large compared to a wavelength,

kpendicularly to the plane of the antenna.

As described in Dyson U.S. Patent 2,958,081, issued on February 11,1960, the balanced equiangular spiral antennas described above can bemodified to exhibit unidirectional radiation patterns while maintainingthe broad bandwidths which such antennas possess. The unidirectionalradiation patterns are achieved by wrapping a planar balancedequiangular spiral antenna on the surface of a cone and feeding theantenna at the apex thereof by a feed cable carried along one arm. Suchantennas exhibit substantially symmetric radiation patf terns having amaximum on the antenna axis off the apex of the cone.

In accordance with the invention it has been found that it is possibleto obtain a conical beam mode of operation from the balanced conicalantenna by constructing the antenna with an even number of arms morethan 'two and connecting these arms to provide a suppression of theradiated fields on the axis of the antenna. The construction of theantennas of the invention will be better understood from the followingdetailed description thereof taken in conjunction with the accompanyingdiagrams in which:

FIGURE 1 is a plan View of an antenna embodying the features of theinvention; 'f FIGURE 2 is an enlarged detail of the apex of the antennashown in FIGURE 1;

FIGURE 3 is an enlarged detail looking down on the apex of the antennaof FIGURE 1;

FIGURE 4 is a sketch of the coordinate system used inr obtainingradiation patterns for the antennas of the invention; and

g'Jnitecl States Patent i.

Btlli Patented June 8, 1965 FIGURES 5(51), 5(b), and 5 (c) are typicalradiation patterns of the electric fields in the planes indicated.

FIGURE 1 depicts a typical embodiment of the invention comprising fourspiraling arms 11, 12, 13, and 14 which are wound on the surface of anelectrically non-conductive cone 16. As shown in FIGURE 2, a smallportion of the cone near the apex is truncated, since it would bephysically impossible to construct this portion of the antenna becauseof the extremely small size of the arms in this region.

FIGURE 3, a top view of a portion of the antenna of FIGURE 1, shows aprojection thereof on a plane surface. In such a projection each of thearms of the antenna is defined by an equiangular (or logarithmic)spiral. An equiangular spiral is a plane curve which may be defined bythe equation:

where p and g5 are the conventional polar coordinates and a and k arepositive constants.

The equiangular spiral curve derives its name from one of itsproperties, namely, the fact that the angle (designated a) formed by aradius vector and a tangent to the curve at the point of intersectionwith the radius vector, is always constant. The value of a can bedetermined from the relationship The constant a thus determines the rateof spiral of the curve and the constant k determines the physicalstarting point of the curve when 95:0, as will be apparent to thoseskilled in the art.

Considering the plane curves partially shown in FIG- URE 3 (that is, theprojection of a portion of the outlines of the arms of the actualantenna), the outer and inner edges of every arm are defined by the samecurve which is rotated about the origin or central point of the figure.Thus, for example, consider the outer edge of arm Il. This curve can berepresented by an equation of the form The inner edge of arm Il isactually the same curve as that defining the outer edge, except that ithas been d1splaced about the originy through an angle The equation ofthe inner edge of arm 11 may therefore be represented by pz-:k-lzK/)lwhere In a similar manner the curves defining arms I2, 13, and I4 areidentical with those defining arm 11 but have been rotated throughconsecutive angles of (1r/2 radians) and thus have the equations ap--p3= ke 2 for the outer edge of arm 12 and ane-a for the inner edge ofarm I2,

ansi-'t is the ilusieione angle.

The. inner @des ,Qiarm ll'isderiefibya Y Y F [gebot-) Kip! where YK/:e-bt n v f The yedges of the otherrarms (eg. 12) can be defined bythe equations of the same type:

` #sme-'bz2 (outer :edge of arm 1 2), A and (inner ,edgeof arm 12).etc-i Y It can be .Ser-i1 that, for an included Cone angle tlf, .of

180 (Leitha planar' ferm Qf the antenna) the above equations reduced tothe form j 91:19, 'P2=KP1 etc. as previously described.,`

, "In contrastA to the balanced two-arm version-in which the arms arfefed 180 out of phase, with the antennas of the invention the number ofchoices ofV feeding systems increases. lIn general, l-theantenna canbe-'fed in anyA symmetrical mannerV which provides a supression Yo f theradiatedriields on the axis .of thefantenna.y VFor fexample, with afour-arm antenna this' can be achieved by connecting 'opposite armstogether and feedingone pair againsttheother, i.e., 1180 Yout of phase.Such a sys- Vtern is shown in FIGURES 2 and 3, which `show a coaxial'Hatteras a'rfefdsiredf Forthis reason .each Qfthe arms ris providedwith a VdummyY coaxialjcablel forpreserving .A physical symmetry. As analternative to thisr method ofY feeding, the antenna can befedbyi-arbalanced feed' line,

or a coaxial-line and balun, placed on the axis of sym- Thefour-armantenna can valso be fed in accordance with the invention byexciting thentw'o pairs of arms with a 9.0 phase shiftl b'etweenthemi.Thus, forV example, if arm 11 of FIGURE 3 isiused as a datum,suppression of radiation along the antenna axis will be obtained if theexcitations to the arms have the following phase relationships: Y

Aim 11 0? Arm 14 90 Arm 13 180v Arm 12 v 270 Antennas having more than4, eg., 6 arms, ca nibe'excited in methods similar to those describedabove. In

oneme'thod a uniform delay Y is introducedV progressively betweenadjacent-armsf'amounting to 60 for a six-arm antenna. (In the generalcase of anvantenna having n arms, the phase change between successivearms would be where n is the number of arms.) Thus if one arm is takenas the datum andfas'suming that the arms spiral outwardly in a clockwisedirection when viewed from the top or apex of the cone, successive armsproceeding inra counand isV usedzonlyto support the arms, it can ibeeliminated ter clockwise directionl would have the phase relationships:0,'-"60, '-120", -.180, -240, -300. If thearms spiraled in the oppositedirection to that assumed, lthe relative phaseV shift would alsoA bevreversed and appliedtorthe arms in a clockwise direction.y

In another suitable lmethod' forfeeding an antenna of the invention, thephase shifts V1.80,? ebetweenfarms, In this c ase, for a six-armLantenna successive arms would have the phase relationships; 0"',-k-l'80, 0, -180, 0, V-'l80. This manner of feeding also can be appliedgenerallyto an `antenna `having any even number of arms.

Although in general antennas of the invention with Yonlyfour arms/willrbe'satistactory-.for many practical purposes, a greater numberofa-rms'can'be used-to provide more uniform pattern Vand far`iield'phase characteristics as a function ofthe azimuthalanglee (FIGURE4). As shown in FIGURE lythe antennas of the-,invention do not radiateinan axial direction, so thatthe radiation patternis conical in shape.By suitable selectionlof the Yantenna parameters, particularly the angleor governing the rate of spiral, the angle of orientationof the maxi-VVV'mum radiation off the axis of `the antennaY can be made to assumevalues over a widerangel. Thus, for example, the angle of orientation 0lofi the beam maximum can be varied through the range from about 40 tomore than 90 -by yvarying or from about 75" to about 45', for coneshaving a cone angle ,gl/,of about 15720". When the beam maximum rislocated at 0:90", the antenna lconstitutes a simple, very broad band,eircularly polarized, omnidirectionalradiation source. f e A Thearms ofthe antenna are made of anelectrically conducting material, suitablysheet copper, aluminum, or the like. The antennas canbe `constructed inany suit- Yablemanner.` A preferred method is by drawing the outline ofthe arms on the development of the leone, which drawingisthentransferred by a vsilk screen lprocess to a thin copper-clad Teflonimpregnated` glass cloth. After 'forming-the Yarms by etching awaytheiurndesiredr portions of the copper cladding, the base material isformed into a cone and Ithe arms are soldered along the joint. The

cone thus formed can be supported by any ofthewellknown materials whichvare eicient insulators Aat high frequencies, suchasn polystyrene.-Since the coneA of insulating material is not anv essential partof theantenna if if the arms are madelof a'rigid material, such as a sheet @ofcopper stnongfeno'ugh to support its ownuweight.

" The` vupper. frequency limitrof Athe 4band/ofoperation for the.antennasoof the` invention is determined by the '.nenessofl theconstruction of the yspiral atathe feedpoint, .i.e.,'atthefvertex of thecone. .i Since equiangular lspiral curves, converge toa point as alimit-atthe origin, it is necessary in a .practical structure ytoterminate the central portion ina small straight or tapered section'.The 4upper -cut-off frequency ofV the antenna .is kthe frequency atwhich ',the'truncated apex of thecone.ibecomes'approximately 1Awave-length in diameter. As an example, for a termination` vof the Yapexat a I-inChdameter the upper frequency limit isapproximately-30004000mc..

` The operatingbandwidth is at the control of the designer. The lowfrequency limitsf'are effected by the length and thefwid'th of thearmsof the antennas and the low frequency limit can be lowered by increasingthe arm ylength and/ or by increasing the width of the arms as in thecase of the balanced two-arm conical antennas.

' Although in the preferred form the antennas of the invention have armseach edge of which is defined by an equiangular spiral, so that thewidth of each arm constantly increases at increasing distances from theapex of the cone, it is also possible to construct practical antennashaving advantageously wide `bandwidths in which the width of the arm isconstant or essentially constant. Thus, for example, antennas in whichthe arms consist of y coaxial cables alone, arranged to followequiangular spiral paths, may have acceptable patterns, particularly ifa is large, e.g., 60 or more. These wire versions of the antenna canmost conveniently be constructed from rigid wall coaxial cable and areadvantageous for use at the UHF and VHF frequencies.

It is also possible to construct practical antennas havingadvantageously wide bandwidths in which the Iarms are constructed tofollow an Archimedes spiral, which is defined by the equation Althoughthese antennas are not frequency independent, they also produce conicalbeam or omnidirectional patterns with the excitations previouslydescribed. For

antennas based on the Archimedes spiral the angle between the axis ofthe antenna and the maximum of radiation varies with the frequency ofoperation.

The practical results which are obtainable with the kantennas of theinvention are demonstrated by a four arm antenna constructed on a 15coney with a diameter of 31 cm. at its base and further defined by theparameters a=45 and K=0.925. This antenna was etched from flexible,copper-clad, Teflon-impregnated, fiberglass material and then formedinto a cone. The feed cable was RGl4l/U. The energized cable was carriedalong one arm; dummy cables were on the other arms to maintainstructural symmetry. Typical radiation patterns of this antenna areshown in FIGURE 5 for a frequency of 1200 mc. The patterns shown are forE, and E, polarized fields. FIGURES 5(a) and 5 (b) are pattern cutsthrough the axis of the antenna at right angles to each other; FIGURE 5(c) is for a cut perpendicular to the axis, on the 0=90 plane.

Antennas in accordance with the invention can be made to have bandwidthsof -t-o 2O or more to 1 over which the radiation patterns and inputimpedance are essentially constant. In order to obtain the maximumbandwidth, however, it should be noted that these are balanced antennasand a balanced feed is necessary for optimum performance. The feed maybe brought in along the axis of the antenna by using a balanced feedline or by an unbalanced line and balancing transformer or balun. The

bandwidth of this latter method, of course, depends upon the bandwidthof the balun. The rapid decay of the current along the arms, however,makes possible the previously mentioned highly useful method of feedingthe balanced antenna with an unbalanced transmission line. Since theends of the antenna arms do not carry appreciable antenna currentsexcept at the very lowest frequency of operation, the arms themselvesact as an infinite balun, the feed terminals are isolated from ground ina balanced manner and the outside of the feed cable beyond the antennaarms does not carry a significant amount of antenna current. However, asthe frequency Iof operation is decreased a point will be reached wherethe presence and location of this cable alters the radiation pattern..This frequency, however, is below that at which the antenna should beexpected to operate satisfactorily. In order to compensate for thepresence of the feed cable insofar as possible, a dummy cable may beplaced on the other arm to maintain physical symmetry.

The input impedance ofthe antennas of the invention remains relativelyconstant over a wider frequency range than the usable pattern bandwidth.

The foregoing detailed description has been given for clearness ofunderstanding only, and no unnecessary limitations should be understoodtherefrom, as modificattions will be obvious to those skilled in theart.

What is claimed is:

1. A broadband antenna comprising an even number greater than 2 ofsubstantially identical electrically conducting elements wound so as tolie on a conical surface having an included cone apex angle in the rangebetween approximately 15 and 20, at least one of said elements beingelectrically insulated from the remainder, each of said elements havingat least one edge lwhich projected on a plane perpendicular to the axisof said conical sur-` face is substantially in the form of a spiral,said elements being symmetrically placed about the axis of said conicalsurface.

2. The antenna of claim 1 wherein said spiral is an equiangular spiral.

3. The antenna of claim l wherein the number of said arms is 4.

4. The antenna of claim 1 which is fed at the apex by a feed cable whichis carried on 4one of said elements.

5. The antenna of claim 4 in which a dummy feed cable is carried by eachof said other elements.

6. A broadband antenna comprising an even number greater than 2 ofsubstantially identical electrically conducting elements wound so as tolie on a conical surface having an included cone apex angle in the rangebetween approximately 15 and 20, at least one of said elements beingelectrically insulated from the remainder, each of said elements beingdened by a pair of curves which when projected on a plane perpendicularto the axis of said conical surface have the form of equiangularspirals, said elements being symmetrically placed about the axis of saidconical surface.

7. The antenna of claim 6 which is formed of thin electricallyconducting sheet material.

8. The antenna of claim 6 wherein the number of said arms is 4.

9. The antenna of claim 6 which is fed at the apex by a feed cable whichis carried on one of said elements.

10. The antenna of claim 9 in which a dummy :feed cable is carried byeach of said other elements.

References Cited by the Examiner UNITED STATES PATENTS 2,640,928 6/53Kandoian 343-908 2,958,081 10/ 60 Dyson 343--895 2,990,548 6/61 Wheeler343--895 3,019,439 6/62 Reis et al 343-895 X OTHER REFERENCES ResearchStudies on Problems Relating to E.C.M. Antennas, AF33 (616), 3220,Report No. 9 published- Wright Field, Feb. 20, 1958.

HERMAN KARL SAALBACH, Primary Examiner.

GEORGE N. WESTBY, ELI LIEBERMAN, Examiners.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,188,643 June 8, 1965 John D. Dyson et al.

It is hereby certified that error appears in the above numbered patentrequiring correction and that the said Letters Patent should read ascorrected below.

Column 2, lines 32 and 33, the equation should appear as shown belowinstead of as in the patent:

t -i an Ct-a same column 2, lines 67 and 68, the equation should appearas shown below instead of as in the patent:

Signed and sealed this 2nd tiny of November 1965.

(SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Attesting Officer Commissioner ofPatents

1. A BROADBAND ANTENNA COMPRISING AN EVEN NUMBER GREATER THAN 2 OFSUBSTANTIALLY IDENTICAL ELECTRICALLY CONDUCTING ELEMENTS WOUND SO AS TOLIE ON A CONICAL SURFACE HAVING AN INCLUDED CONE APEX ANGLE IN THE RANGEBETWEEN APPROXIMATELY 15* AND 20*, AT LEAST ONE OF SAID ELEMENTS BEINGELECTRICALLY INSULATED FROM THE REMAINDER, EACH OF SAID ELEMENTS HAVINGAT LEAST ONE EDGE WHICH PROJECTED ON A PLANE PERPENDICULAR TO THE AXISOF SAID CONICAL SURFACE IS SUBSTANTIALLY IN THE FORM OF A SPIRAL, SAIDELEMENTS BEING SYMMETRICALLY PLACED ABOUT THE AXIS OF SAID CONICALSURFACE.